The sensitivity of coupled neutron-photon transport simulations to the underlying nuclear data is studied for oil well logging applications using the Monte Carlo radiation transport code MCNP4C. Results obtained with the JEF-2.2, ENDF/B-VI.5, and JENDL-3.2 data libraries reveal large discrepancies, confirming immediately the importance of well-established nuclear data. In order to refine this conclusion, the impact of several nuclides is determined by varying their neutron and photon total cross sections. As a next step, neutron cross sections and photon production cross sections are modified per reaction channel to identify the most important nuclear reactions playing a role in C/O logging. The influence of neutron and photon angular distributions is studied as well. The outcome of this analysis is a list of nuclear reactions that have a significant impact on borehole logging tool simulations in different environments and, therefore, would deserve much attention in the construction of a new library for oil well logging.